Skip volume snapshot tests if feature is not enabled

Not all cinder volume backends support volume snapshot. For instance,
the default NFS backend doesn't. This patch adds a new entry to
CONF.volume_feature_enabled to add the possibility to skip volume
snapshot tests.

Change-Id: I40ce17924bbe4eb6f1c941df974bfa832ec2a999
diff --git a/tempest/scenario/test_volume_boot_pattern.py b/tempest/scenario/test_volume_boot_pattern.py
index e89ea70..faca31f 100644
--- a/tempest/scenario/test_volume_boot_pattern.py
+++ b/tempest/scenario/test_volume_boot_pattern.py
@@ -35,6 +35,12 @@
      * Boot an additional instance from the new snapshot based volume
      * Check written content in the instance booted from snapshot
     """
+    @classmethod
+    def setUpClass(cls):
+        super(TestVolumeBootPattern, cls).setUpClass()
+
+        if not CONF.volume_feature_enabled.snapshot:
+            raise cls.skipException("Cinder volume snapshots are disabled")
 
     def _create_volume_from_image(self):
         img_uuid = CONF.compute.image_ref